The conventional AC-DC converter, for example, PFC (Power Factor Correction) converter, comprises a bridge rectifier and a boost converter. FIG. 1A schematically shows a prior art PFC converter. The PFC converter comprises a main circuit 101 and a control circuit 102. The main circuit 101 comprises a bridge rectifier and a boost converter. The bridge rectifier comprises diodes D1˜D4. An AC power supply having a first terminal “L” and a second terminal “N” is supplied to the input terminals of the bridge rectifier. Output terminals of the bridge rectifier are coupled to input terminals of the boost converter. The boost converter comprises an inductor L1, a power switch S1, a rectifier D5 and an output capacitor CO. The boost converter provides an output voltage VO at an output terminal. A load LED is coupled between the output terminal of the boost converter and one terminal of a resistor RL. The voltage at the common node F of the LED and the resistor RL is UF.
The control circuit 102 comprises a first loop, a second loop, a multiplier and a driving circuit. The first loop comprising a dividing circuit 103 and a circuit 104 is used to control the input current or the inductor current. The dividing circuit 103 comprising resistors R2 and R3 is used to sense the first input voltage VHS1, so as to get a second input signal VHS2 at the common node of resistors R2 and R3. The circuit 104 generates an output signal VHS3 by amplifying or normalizing the second input signal VHS2.
The second loop comprises an amplifier 105 and a compensation network which is consisted by a resistor R1 and a capacitor C1. The second loop is configured to generate a second output signal VO2. A first input terminal of the amplifier 105 is connected to a voltage reference REF while a second input terminal of the amplifier 105 is coupled to the output terminal of the PFC converter by a low pass filter 106. Due to the existence of the filter 106, the voltage at the second input terminal of the amplifier 105 would be stable even when the common node F has large voltage ripples.
The multiplier 107 is configured to provide a multiplying signal VCOM through multiplying the output signal VHS3 by the second output signal VO2.
The driving circuit comprising a comparator 108 receives the multiplying signal VCOM and a current sampling signal CS, and generates a driving signal VDRV to turn on/off the power switch S1.
Generally, a large output capacitor CO is used to get a stable output voltage VO that may be as high as 400V. A large capacitor that is able to withstand a high voltage is not only expensive, but may be easily damaged. A smaller output capacitor CO is preferred, but will cause a large ripple on the LED current.
For a typical PFC converter as shown in FIG. 1A, due to power factor correction, the first input current IHS1 and/or inductor current signal IL1 have a same frequency and phase with the first input voltage VHS1. The first input voltage VHS1 can be expressed asVHS1=A|Sin(2πωt)|  (1)wherein A is the amplitude of the first input voltage VHS1. | | represents the absolute value, and ω is the angular frequency. Assuming that the amplitude of the inductor current IL1 is B, then the inductor current IL1 could be expressed as,IL1=B|Sin(2πωt)|  (2)
Assuming that the conversion efficiency is close to 100%, according to the power conservation law, we can getVHS1×IL1=VO×ILED  (3)
The LED current ILED could be expressed as,
                              I          LED                =                                                            V                                  HS                  ⁢                                                                          ⁢                  1                                            ×                              I                                  L                  ⁢                                                                          ⁢                  1                                                                    V              0                                =                                    A              ×              B              ×                                                Sin                  2                                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    ω                    ⁢                                                                                  ⁢                    t                                    )                                                                    V              0                                                          (        4        )            
VO could be regarded as a constant since the voltage across the LED would not vary obviously with the LED current. Then, the LED current ILED is a square of a sinusoidal function. The ratio between the peak value and the average value (PAPR) of the LED current ILED is 2. This will reduce LED life greatly. According to the equation (3), the input power of the Boost converter could be maintained substantially constant if the inductor current IL1 expressed as
                              I                      L            ⁢                                                  ⁢            1                          =                  C                      Sin            ⁡                          (                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                t                            )                                                          (        5        )            can be obtained. The sin function in the equations (1) and (5) could be counteracted. Accordingly, as shown in FIG. 1B, the LED current ILED would remain substantially constant. However, the inductor current shown in equation (5) will cause the power factor of the PFC converter to be particularly low. So, converters and methods that could get a stable LED current and also a high power factor are required.